During the past decade, many sensing and signaling components of abiotic stress signaling pathways have emerged. Such pathways play primary roles in reacting to short-term or "emergency" insults brought about by rapid changes in environmental conditions. More important, stress sensing and signaling pathways set into motion "intermediary" responses that lead to important, long-term adaptations for abiotic stress tolerance involving changes in plant development, growth dynamics, vegetative-floral meristem transitions, and seed production and maturation programs. Such long-term adaptations have perhaps the greatest impact for engineering the retention or improvement of agricultural productivity under abiotic stress conditions. Many sensing and signaling pathway components in plants, such as those that participate in MAP kinase and phosphatase cascades, have counterparts in animals and yeast (33,64,125-128). For example, a plant MAP kinase homologue effectively replaced the yeast HOG1 kinase under high osmotic stress conditions (129). Other signaling components are unique to plants, such as calcium-dependent protein kinases (130,131).
A histidine kinase has been identified as an osmosensor in Arabidopsis (132), which shares homology with other eukaryotic two-component response regulators. Stress-dependent regulation of an increasing number of elements in several signaling pathways has been reported (133-136). These pathways apparently function at different levels, are interconnected, and respond differentially to a variety of environmental stimuli through a series of signaling molecules including calcium, abscisic acid (ABA) (137), gib-berellins (138), ethylene, phosphatidylinositols, cytokinins (38), brassinos-teroids (139), or jasmonates (12,134,136,140-145).
Differences in how stresses are perceived and how such information is processed into biochemical responses between, for example, glycophytes and halophytes are likely to account for tolerance phenotypes. Halophytes may have evolved distinct stress recognition or signaling pathways and regulatory controls to confer stress protection. For example, the halophyte Aster tripolium possesses an Na+-sensing system that down-regulates K+ uptake by guard cells in response to high salt concentrations (146). This results in stomatal closure and prevents excessive Na+ uptake via the transpiration stream. In contrast, nonhalophytic relatives appear to lack this specialized sensing ability and may actually respond to Na+ ions by increasing stomatal apertures. Assuming that the Arabidopsis genome contains a gene complement that is largely similar to that present in the genome of the halophytic Mesembryanthemum, differences in stress perception and signaling may be explained by regulatory changes that evolved as a result of duplication and alterations in genes encoding signal transduction components (e.g., receptors, protein kinases, and phosphatases). Other distinctions may be the result of differences in the numbers and specificity of transcription factors and the cw-regulatory elements they recognize.
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